In general, with the development of recent optical technologies as their springboards, display technologies using various processes are being suggested and commercialized and Plasma Display Panels (PDPs) and Liquid Crystal Displays (LCDs) are replacing the cathode-ray tubes of the related art. Required characteristics of polymer materials for these displays are being further advanced. For example, particularly important issues in the case of LCDs have been wide viewing angles, high contrast ratios, uniformity of screen displays and the inhibition of image color variations according to viewing angles as reductions in thickness and weight as well as the enlargement of LCD screens are being pushed ahead.
Accordingly, a diversity of polymer films such as polarizing films, polarizer protection films, retardation films, plastic substrates, and light guide plates have been used, and LCD devices of various modes using twisted nematic (TN), super Twisted nematic (STN), vertical alignment (VA), and in-plane switching (IPS) liquid crystal cells have been developed as liquid crystals have also been developed. All of these liquid crystal cells have intrinsic optical anisotropy properties due to their intrinsic liquid crystal alignments, and films to which retardation functions have been given by stretching various types of polymer films have been suggested to compensate the optical anisotropic properties.
Specifically, since high birefringence characteristics and orientations possessed by liquid crystal molecules are used in LCD devices, refractive indexes of LCD devices are varied according to viewing angles thereof, and the color and brightness of the screens of LCD devices vary accordingly. For instance, since most of the liquid crystal molecules have positive retardations in the thickness direction of the liquid crystal display surface, compensation films having negative retardations in the thickness direction of the liquid crystal display surface are needed to compensate for the positive retardations. Further, although light cannot pass through two polarizers crossing at right angles to each other, a light-leakage phenomenon may occur, since optical axes of the two polarizers do not cross at right angles to each other if the two polarizers are tilted. Therefore, a compensation film having a surface directional retardation is needed to compensate the light-leakage phenomenon. Additionally, display devices using liquid crystals need to have a surface directional retardation as well as a thickness directional retardation compensated in order to enlarge the viewing angle.
It is necessary that the birefringence of retardation compensation films should be easily controlled. However, the birefringence of films not only includes a fundamental birefringence of a material itself, but is also formed by the orientation of polymer chains in the films. The orientation of polymer chains is mostly occurs coercively due to force applied from the outside or is caused by intrinsic characteristics possessed by the material, and a method for molecular orientation through external force comprises stretching a polymer film uniaxially or biaxially.
In order to solve LCD viewing angle problems due to the foregoing intrinsic birefringence properties of liquid crystals, N-TAC, V-TAC, and COP films have recently been used as compensation films or retardation films. However, such films have disadvantages in being relatively expensive and having complicated preparation processes.